This invention relates to the field of graphite or other carbon electrodes for electric furnaces and like equipment in which the electrodes are consumed in use and continuously fed into the furnace to accommodate such consumption. Such electrodes typically range from 12 to 28 inches (30 to 71 cm) in diameter and from 60 to 110 inches (152 to 279 cm) in length. Electrodes at the upper end of this dimensional range may weigh as much as 4,000 lbs (1,810 Kg). Each end face of each electrode section contains a threaded socket, normally frusto-conical in configuration, to accommodate one end of a double frusto-conical threaded connecting pin. Such pins are commonly referred to in the industry as nipples. By screwing an electrode section on each end of a nipple, successive electrode sections are joined to provide for continuous furnace operation.
In making the above described electrode joint, one end of the nipple is screwed into place in the electrode socket to the approximate center of the nipple measured along its longitudinal axis. This assembly of electrode and nipple with half of the nipple projecting from the end face of the electrode section is then threaded into the empty socket on the end of the preceding electrode section until the end faces of the electrode sections are in contact. With a tapered nipple, its largest diameter is at its longitudinal center which should also be in the plane of contact of the electrode section butt ends.
Since the electrode sections are screwed up on the nipple until the electrode section butt ends are in firm contact, each engaged thread in the assembly has a load bearing flank and a non-load bearing flank. In the nipple threads the thread flank nearer the axial center of the nipple is always the load bearing flank in the completed assembly. Conversely, the flank of each electrode socket thread nearer the axial center of the nipple is the idle flank. The nipples are not screwed into contact with the bottom of the socket.
Because of the great weight of the depending electrode, the compressive stresses imposed by tightening the electrode sections together, the mechanical stresses imposed by furnace operation and the great additional stresses caused by thermal expansion and/or contraction during heating and/or cooling of the assembly in service, the strength of the joint and, in particular, the strength of the nipple are recognized to be critical.
Much attention has been devoted in the past by those skilled in this art to minimizing the effects of this inherent problem. Important in this is recognition by the art of the adverse consequences of assymetric assembly of the nipple into the sockets of abutting electrode sections. When the nipple is properly "centered", i.e., when the largest diameter of the symmetrical frusto-conical nipple is in the plane of the abutting ends of adjacent electrode sections, the thread design of the nipple and socket provides sufficient clearance on the idle flank to accommodate thermally induced dimensional change without creating excessive additional stresses. However, if the nipple is not centered the clearance on the idle flanks may be reduced or eliminated causing excessive stresses to occur when the electrode is in service. The prior art is replete with various suggested expendients for minimizing or overcoming this problem. U.S. Pat. Nos. 2,957,716; 2,970,854; 3,048,433; 3,084,419; 3,088,762; 3,134,616; 3,140,967; 3,517,954 and 3,517,957 all relate directly or indirectly to the problem of centering the nipple and maintaining evenly distributed thread clearance in electrode joints. In general "centering" was accomplished in the prior art in one of two ways. One way was to provide a stop on the thread or at the base of the electrode socket that would physically limit the depth to which the nipple could be screwed into the socket of the first electrode section. In the heat of the furnace this stop would fuse or deform to provide a clearance on the idle thread flank. Any such system is inherently expensive and only as good as the manufacturing tolerances that can be maintained. A mechanical stop can only insure centering if the socket, the socket and nipple threads and the stop device are manufactured to precise dimensions. The other approach in the prior art is to insert the nipple in the first electrode socket only until its equator is in the plane of the electrode end. This can be accomplished by mechanical or optical gauging. The nipple is then locked in this position by some sort of hard locking cement or a rigid metal or graphite pin traversing the interface between nipple and electrode socket to prevent rotational movement between the two. Often the locking cement was designed to carbonize in the furnace and further prevent rotation of the nipple in the socket.
Traditionally it has been the practice to assemble a nipple to a first electrode section at the furnace site and before the electrode section was coupled to the preceding electrode section already on the furnace. More recently furnace operators and electrode manufacturers have recognized certain benefits from preassembling the electrode and nipple at the point of electrode manufacture. Not the least important benefit is that the electrode manufacturer can more accurately and consistently "center" the nipple in the manufacturing plant than can realistically be done on the melt shop floor and thereby assure proper clearances on the idle thread flanks. However, if the nipple is not locked in position it has been found to move significantly ("unwind") during the inevitable shipment and handling preceding positioning on the furnace. If, on the other hand, the nipple is carefully positioned and locked in place by any of the many schemes suggested by the prior art it is impossible to remove the nipple either before or after the electrode is put in service. Such removal is often necessary for any one of a variety of reasons arising out of electric furnace practice. In addition, electrodes are normally stored and shipped with their longitudinal axes in a horizontal position. Since the preassembled nipple is not screwed in tightly it will sag or droop and be misaligned when the locking cement sets. This prevents proper contact between adjacent electrode faces when the next electrode section is screwed on to the half of the nipple projecting from the face of the previous electrode section.
It is therefore an object of the present invention to provide for preassembling a nipple to an electrode section in a precisely centered position to preserve the integrally machined idle flank thread clearances and to secure the nipple in such preset position in a manner to (a) prevent accidental displacement in shipment and handling, (b) permit intentional disassembly should that be necessary, (c) permit limited relative movement between the electrode and nipple to facilitate perfect axial alignment in service and (d) leave the joint and thread clearance spaces free of material of any kind when the electrode is in service to thereby permit unrestricted thermal expansion and contraction to the extent provided for by the idle flank thread clearances.